Electric lamp circuit



Dec. 23, 1958 J. w. STRANGE ET AL 2,866,133

ELECTRIC LAMP CIRCUIT 1 Original Filed Sept. 20, 1949 3 Sheets-Sheet 1 61 3 I M 1 E In... v m PL 1 1L. 4 w w 5 W T F m Fl 6 W w 1 W F4 I" B 1958J. w. STRANGE ETAL 2,863,133

ELECTRIC LAMP CIRCUIT Original Filed Sept. 20. 1949 3 Sheets-Sheet 3 5/2 E3 5 k T T4 7b V I w T "a I 2 I I a 1 52 4 Cl b m E A++ arm u,

United States Patent ELECTRIC LAMP CIRCUIT John William Strange, DouglasThomas Waigh, and William Robert Bloxsidge, London, England, assignorsto Thorn Electrical Industries Limited, London, England, a Britishcompany Original application September 20, 1949, Serial No. 116,756, nowPatent No. 2,683,240, dated July 6, 1954. 2424322 and this applicationJuly 2, 1954, Serial No.

4 Claims. (Cl. 315-95) This invention relates to circuits use to controlthe brightness of low-pressure discharge lamps of the hotcathode type. Alow-pressure discharge lamp is one which operates normally at atemperature which exceeds the ambient temperature by not more than 60 C.

Filament lamps may be fed through adjustable dimming means capable ofreducing the light emission progressively to zero. If coloured light isrequired as, for

example, in theatre lighting, colour filters can be fitted in front ofthe filament lamps. The efiiciency of the combination of filament lampand colour filter is very low. The use of fluorescent discharge lampshas been suggested for colour lighting, since, by suitable choice of theluminescent material, a wide range of colours can be produced withoutthe use of filters. The disadvantage of such lamps is that with normalcircuits, consisting of a lamp in series with a choke and thefilamentary lamp electrodes connected in series for starting by a glowor thermal switch, only a limited range of brightness is available.Attempts to use low voltages result in erratic and intermittentoperation.

An object of this invention is to provide brightness control means whichwill permit substantial dimming of.

low-pressure discharge lamps of the hot-cathode type, and especiallyfluorescent lamps.

Another object of the invention in some of its forms is to provide anelectric lamp system which can be arranged to give a substantiallysmooth decrease of brightnessover the range from full brightness to lessthan onehundredth of full brightness.

A further object is to provide an electric lamp system in which thebrightness of two or more lamps may be varied simultaneously by a singleoperators control.

According to this invention, an electric lamp circuit includes a lampand ballast combination consisting of a low-pressure electric dischargelamp of the hot-cathode type having two electrodes, and a ballastimpedance element connected in series with the lamp and adapted to limitthe total current flowing between the electrodes, means for supplyingheating current continuously to each of the electrodes of the lamp atleast over the lower part of the operating range of brightness of thelamp, and means for applying between the terminals of the saidcombination a potential difference which is substantially continuouslyvariable by a dimmer control member while the lamp is operating, bothsaid means being adapted to be supplied from a common source of power.According to the invention in a preferred particular aspect, the saidmeans for applying the potential difference which is variable while thelamp is operating are a variable series impedance element. Thisimpedance element may be a variable resistor or a variable saturablereactor, or a variable resistor in series with a variable saturablereactor.

A filament lamp may be connected in parallel with the discharge lam thearrangement being such that, in the operating range within which thepotential difference across the lamps is insufiicient to cause thedischarge 2,866,133 Patented Dec. 23, 1958 There may be provided a lampsystem including two i or more of the lamp circuits hereinbeforespecified as according to this invention connected in parallel andhaving a common dimmer control member.

Two further particular aspects of this invention and three separatearrangements of the third aspect will now be specified.

According to this invention in its second aspect, an electric lampcircuit includes a lamp and ballast combination consisting of twolow-pressure electric discharge lamps of the hot-cathode type connectedin series to form a single discharge path, and a ballast impedanceelement adaptedto limit the total current flowing in the discharge path,means for supplying heating current continuously to each of theelectrodes of the lamps of the said combination at least over the lowerpart of the operating range of brightness of the lamps, and also meansfor providing between the terminals of the said combination a potentialdifference which is substantially continuously variable by a dimmercontrol while the lamps are operating, both said means being adapted tobe supplied from a common source of power, and an impedance elementconnected between the junction of the discharge spaces of the lamps anda point on the circuit, e. g. a terminal of the source, so selected thatthe last-mentioned impedance element will pass a current through atleast one of the lamps. The magnitude of this current should be such asto maintain the discharge at the lower limit of the controllable rangeof brightness of the lamp. A third lamp may be connected in series withthe other two lamps, in which case a further impedance element may beconnected between the junction of the third lamp with the other twolamps and a point on the circuit, e. g. the other terminal of thesource, so selected that the lastmentioned impedance element will passthrough the said third lamp a current sufficient to maintain thedischarge at the lower limit of the controllable range of brightness ofthe lamp. The said means for supplying heating current may include atransformer having its primary winding connected either across thesupply terminals or in parallel with the discharge path. There may beprovided a lamp system including two or more lamp circuits, specified asaccording to the invention in its second aspect, these circuits beingconnected in parallel, and the said potential difference, which isapplied to the lamp and ballast combination referred to above, beingsubstantially continuously variable by means of a common dimmer controlmember, e. g. associated with a common dimmer element of the kind whichvaries the applied voltage, such as a variably-tapped autotransformer.

According to this invention in its third aspect an electric lamp circuitincludes a lamp and ballast combination consisting of a low-pressureelectric discharge lamp of adapted to limit the total current flowingbetween the electrodes, means for applying between the terminals of thesaid lamp and ballast combination a first potential difference which issubstantially continuously variable by a dimmer control while the lampis operating, and means for applying between the terminals of eachelectrode a second potential difference which is substantiallycontinuously variable by said dimmer control up to a predetermined valuewhile a relatively low value of said first potential difference (i. e.lower than the value of the striking voltage of the lamp) is appliedbetween the lamp electrodes, and for preventing said second potentialdifference from substantially exceeding said predetermined value whilesaid first potential difference is increased beyond said relatively lowvalue.

There may be provided a lamp system including two or more lamp circuits,specified as according to the invention in its third aspect, connectedin parallel and having a common dimmer control.

In a first arrangement according to the invention in the said thirdaspect, an electric lamp circuit includes a lamp and ballast combinationconsisting of a low-pressure electric discharge lamp of the hot-cathodetype having two electrodes, and a ballast impedance adapted to limit thetotal current flowing between the electrodes, dimmer means for providingbetween the terminals of the said combination a potential dilferencewhich is substantially continuously variable by a dimmer control memberwhile the lamp is operating, and means for supplying to each of theelectrodes of the lamp heating current which is substantiallycontinuously variable by a regulator while a relatively low value ofpotential difference is applied between the said electrodes, both meansbeing adapted to be supplied from a common source of power. The dimmercontrol member and the regulator may be ganged. There may be provided alamp system including two or more lamp circuits specified as said firstarrangement having a common regulator and a common dimmer control memberactuating a dimmer element, e. g. a variablytapped autotransformer, ofthe kind which varies the applied voltage. The common regulator and thecommon dimmer control member may be ganged.

In a second arrangement according to the invention in the said thirdaspect, an electric lamp circuit for use with an A. C. power sourceincludes a dimmer element in the form of a variably-tappedautotransformer connected between the terminals of the power source andadapted to provide substantially continuously variable voltages betweenthe terminals of a lamp and ballast combination consisting of alow-pressure electric discharge lamp of the hot-cathode type, and aballast impedance connected in series with the lamp and adapted to limitthe total current flowing between the lamp electrodes, said dimmerelement also being connected to feed through a capacitor the primarywinding of a second transformer having separate secondary windingsconnected respectively across the lamp electrodes, and the arrangementbeing such that for a value of the potential difference applied betweenthe lamp electrodes slightly below the value of the striking voltage ofthe lamp the circuit is resonant at the frequency of the power source.

There may be provided a lamp system including two or more lamp circuitsspecified as said second arrangement connected in parallel and having acommon dimmer element.

In a third arrangement according to the invention in the said thirdaspect, an electric lamp circuit for use with an A. C. power sourceincludes a dimmer element in the form of a variably-tappedautotransformer connected between the terminals of the power source andadapted to provide substantially continuously variable voltages betweenthe terminals of a lamp and ballast combination consisting of alow-pressure discharge lamp of the hot-cathode type connected in serieswith a ballast impedance element which is adapted to limit the totalcurrent flowing between the lamp electrodes, said dimmer element alsobeing connected to feed the primary winding of a voltage limitingtransformer, and the circuit also including a second transformer havinga primary winding supplied through the secondary winding of said voltagelimiting transformer and separate secondary windings connectedrespectively across the lamp electrodes.

There may be provided a lamp system including two or more lamp circuitsspecified as said third arrangement, connected in parallel and having acommon dimmer element and a common voltage limiting transformer.

In all the above-mentioned circuits, when supplied from an A. C. powersource, the means for supplying heating current to the lamp electrodesconsist preferably of a transformer of the leakage-field reactance type.

In all the above-mentioned circuits an external electrode adapted to bemaintained at a fixed potential is preferably placed close to thedischarge lamp envelope, at least in the neighbourhood of the lampelectrodes, in order to increase the ionisation of the spaces adjoiningthese electrodes before the lamp strikes.

Preferred circuit arrangements embodying the invention will now bedescribed, by way of example, with reference to the accompanyingdrawings, each figure of which shows a lamp system for use with an A. C.power source and consisting of two lamp circuits connected in parallel.The mains voltage is 230 unless otherwise stated. In the drawings:

Figure 1 shows a lamp system according to the invention in the saidfirst aspect and in which each lamp circuit includes two discharge lampshaving continuously heated electrodes and connected in series to form asingle discharge path; the circuits have separate adjustable electricalcomponents acting as dimmers and ganged for operation by a commoncontrol member.

Figure 2 shows a lamp system according to the invention in which eachlamp circuit includes a single discharge lamp having continuously heatedelectrodes and in which a filament lamp is connected in parallel witheach discharge lamp; the circuits have a common adjustable electricalelement acting as a dimmer.

Figure 3 shows a lamp system illustrating the invention in the saidsecond aspect and in which the lamp circuits are provided with a commonadjustable electrical element acting as a dimmer.

Figure 4 shows a lamp system similar to that shown in Figure 3 exceptthat the lamp circuits have separate but ganged adjustable electricalelements acting as dimmers.

Figure 5 shows a lamp system illustrating the first arrangement of theinvention in its third aspect; the two lamp circuits have two commonadjustable electrical elements acting respectively as a dimmer and as aregulator.

Figure 6 shows a lamp system similar to that shown in Figure 5 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

Figure 7 shows a lamp system illustrating the second arrangement of theinvention in its third aspect; the lamp circuits have a commonadjustable electrical element acting as a dimmer.

Figure 8 shows a lamp system similar to that shown in Figure 7 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

Figure 9 shows a lamp system illustrating the third arrangement of theinvention in its third aspect; the lamp circuits have a commonadjustable electrical element acting as a dimmer.

Figure 10 shows a lamp system similar to that shown in Figure 9 exceptthat each lamp circuit includes a single discharge lamp instead of twodischarge lamps connected in series.

In each figure of the drawings elements in one lamp circuitcorresponding to like elements in the other lamp circuit will be givenlike reference characters. All the lamps in the circuits described areof the type having directly heated electrodes.

Referring now to Figure 1, which illustrates the invention in its firstaspect, in each lamp circuit two discharge lamps L and L havingfilamentary electrodes E E and E B, respectively are connected in seriesto form a single discharge path. The terminals of the filamentaryelectrodes are indicated by T (e. g. T T T etc.). The lamps areconnected, in series with a ballast choke B and a dimmer elementconsisting of a variable resistor R between terminals L and N of an A.C. power source. A transformer F of the leakage-field reactance type,has a primary winding W connected between the source VVAAI.

.5 terminals and separate secondary windings W and W connected to supplyheating current to electrodes E and B, respectively. Electrodes E and E,are connected in series with a further separate secondary winding Wwhich supplies them with heating current. External electrodes S and Sconsisting of metallic strips connected to earth are disposed alongsidethe envelopes of their respective lamps L and L The dimmer elements areganged and connected to a common dimmer control member indicateddiagrammatically by P.

Each transformer F is so designed that the electrodes of the lampsheated by the current from the secondary windings W W and W are kept atsuch a temperature that they emit electrons freely. The presence of theadjacent earthed strips encourages ionisation around the electrodes. Astable discharge can be passed between the two electrodes of each lampwith the control set to limit the current to a value as low as 2milliamperes. As the series impedance is decreased, by operation of thedimmer control, the current passing through the lamps increases untilthe lamps reach full brightness.

Thus the brightness of the lamps can be varied from their maximum to alow value without instability by operation of the resistors R Thegauging of these resistors enables the brightness of all the lamps inthe system to be varied substantially simultaneously by actuation of asingle operators control.

This circuit arrangement is particularly suitable for a fluorescent lamphaving an envelope 2 ft. in length and 1 /2 in. in diameter, designed torun with a lamp wattage of 40. A suitable value for each of theresistors R is 130,000 ohms. Measurements made on such a circuit, withlamps having a luminescent coating of manganese activated zinc silicate,show that the light emitted by each lamp increases from 5 to 1300lumens, reaching a peak efficiency of 50 L/watt. Any number of similarlamp circuits may be connected in parallel and their dimmer resistorsganged.

A transformer of the leakage-field reactance type is selected for Fsince in operation the lamp electrodes have a negative impedancecharacteristic. This negative.

impedance characteristic is due to small potential differences whichexist between adjacent portions of the electrode and which cause theestablishment of minor discharges between these adjacent portions.

The arrangement shown in Figure 1 may be modified by replacing thevariable dimmer resistors R by variable saturable reactors.Alternatively each dimmer element may consist of a variable resistor inseries with a variable saturable reactor; this arrangement canconveniently be made such that the reactor controls the brightness fromfull brightness to, say, one-tenth full brightness, the resistorcontrolling the brightness over the remainder of the range.

In another modification of the arrangement shown in Figure 1, ineachlamp circuit the two lamps L and L are replaced by a 5-foot 80-wattlamp, the transformer winding W being omitted. The circuit is designedfor operation on 240 volts, and a filament lamp, designed to operate ona 130-vo-lt supply at 60 watts, is connected in parallel with thedischarge lamp. With this arrangement, increase of the potentialdifference across the lamps will increase the brightness of the filamentlamp until the strike potential of the discharge lamp is reached,whereupon the potential difference across the lamps immediatelydecreases, so that a substantially smooth increase in the brightness ofthe two lamps in combination is achieved through this transition period.

In each of the circuits in the system shown in Figure 2 a single lamp Lis connected in series with a ballast choke B between terminals T and Tof a variably-tapped secondary winding W of a transformer F which actsas a dimmer control and which is common to the two circuits. Theterminals T and T of the primary winding W of the transformer F areconnected respectively 6 to source terminals N and L. A leakage-fieldreactance transformer F has a primary winding W connected between thesource terminals and separate secondary windings W and W connected tosupply heating current to lamp electrodes E and E respectively. Anexternal electrode S is provided.

A filament lamp I I is connected in parallel with each discharge lamp Lto emit light when the potential difference across the discharge lampsis too low to cause them to strike.

The brightness of the lamps may be varied by operating the dimmercontrol member and thus altering the potential applied to the lamp andthe ballast choke in series. A discharge lamp, when in the dischargingcondition, has the characteristic of a constant voltage device, andvariations in the potential applied to the lamp and the ballast choke inseries will cause comparatively small variations in the potentialdifference between the lamp electrodes.

This circuit arrangement is particularly suitable for a fluorescent lamphaving an envelope 5 ft. in length and 1 /2 in. in diameter, designed torun with a lamp wattage of 80. Any number of similar lamp circuits maybe connected in parallel provided that the power rating of the dimmertransformer F is increased to a sufficient value.

In the system shown in Figure 3, illustrating the invention in itssecond aspect, each circuit includes two discharge lamps L and L aballast choke B and a leakage-field reactance transformer F connected inthe manner described with reference to Figure 1. External electrodes Sand S are provided. The lamp and ballast combination is connectedbetween terminals T and T of winding W of a variably-tappedautotransformer F acting as the dimmer. Terminals T and T of the dimmerare connected to terminals N and L of the power source respectively. Tis the terminal of the tapping which constitutes the dimmer control. Thejunction of terminals T and T of electrodes E and E respectively isconnected to the power source terminal N through a second impedanceelement consisting of a resistor R in series with a capacitor C Thevalues of R and C are such that a small glow discharge, representing thelower limit of the controllable range of brightness, is maintained ineither or both of the lamps. When the source terminals are phase andneutral, the second impedance element is preferably connected to theneutral terminal. When the dimmer control is in its position of minimumbrightness, at least one of the lamps of each pair (usually L is in ornear the condition of small glow discharge and all the lamps will beginto increase in brightness at substantially the same rates when thedimmer control is moved fom this minimum brightness position. Thereforea single operator's control may be used despite variations from lamp tolamp of the potential normally required to cause each lamp to strike.

This circuit arrangement is particularly suitable for 2-foot 40-wattlamps, and suitable values of R and C for use with such lamps are100,000 ohms and 5,000 pF, respectively.

In a modification of the arrangement shown in Figure 3, the capacitors Care connected to the tapping terminal T instead of to the neutralterminal N.

Any number of similar circuits may be connected in parallel providedthat a separate second impedance element is provided for each pair oflamps and that the rating of the dimmer transformer is increased to asuitable value.

The lamp system shown in Figure 4 is similar to that shown in Figure 3except that the dimmer for each circuit consists of a resistor Rconnected in series with the lamp and ballast combination between thepower source terminals. These separate resistors R have their tappingsganged by means indicated diagrammatically by P to enable the brightnessof all the lamps to be varied substantially simultaneously by a singleoperators control member. This circuit arrangement is also particularlysuitable for 2-foot 40-watt lamps. A suitable value of R is 130,000ohms.

In the system hereinbefore described the lamp electrodes are heatedcontinuously and when the power applied to each lamp is reduced belowthe level at which a column discharge is visible, the ends of the lampwill still glow owing to the ionisation of the spaces surrounding thehot electrodes. If it is desired to extinguish the lamp completely theelectrode heating currents may be switched off but the desired smoothdecrease of brightness will not then be obtained. Circuits in which thelamps can be completely extinguished will now be described.

The arrangement shown in Figure 4 may be modified by eliminating theconnections shown for each primary winding W and connecting theterminals of each of these windings directly to the terminals T and T ofthe associated lamps.

The lamp system shown in Figure 5, illustrating the first arrangement ofthe invention in its third aspect, is similar to the system shown inFigure 3 except that the primary windings W of leakage-field reactancetransformers F are connected in parallel between terminals T and T ofwinding W of a variably-tapped autotransformer F connected as aregulator. Thus the values of the electrode heating currents may also bevaried. Also the second impedance element (i. e. the resistor R and thecapacitor C is connected to the terminal T of the regulator instead ofto the terminal N of the source, thereby enabling the lamps to becompletely extinguished when the dimmer control and the regulator areset to zero.

The dimmer control and regulator transformers F and F are ganged bymeans of. levers A operatively connected to the tapping selectors G andpivoted at C. The levers are moved by cams J and push-rods K and areheld in contact with the push-rods by compression springs H. The camsare mounted on a shaft M rotatable in bearings V. An operators handle isalso mounted on shaft M. The relative rates of movement of the tappingselectors of the dimmer and the regulator transformers may be altered inknown manner by varying the cam contours.

This circuit arrangement is particularly suitable for 2-foot 40-wattlamps. In order to assist the ionisation of the spaces surrounding theelectrodes before the arc strikes, the minimum value of the potentialdifference applied across each pair of lamps is set at approximately 130volts. This potential difference is applied while the regulator isoperated to increase the electrode heating currents to their maximumvalues, causing the ends of the lamps surrounding the electrodes tobegin to glow and eventually causing the lamps to strike; the electrodeheating current is then maintained substantially constant while thedimmer control is operated to increase the potential difference acrossthe lamps until they have reached full brightness. To extinguish thelamps the sequence of operation is reversed. Thus it is possible toincrease the brightness of all the lamps at substantially the same rateover the full range of Zero to maximum brightness using a singleoperators control.

Any number of similar circuits may be connected in parallel providedthat the ratings of the dimmer and the regulator transformers areincreased to suitable values.

The system shown in Figure 6 is similar to that shown in Figure exceptthat a single lamp is used instead of two lamps connected in series.This circuit arrangement is particularly suitable for SO-Watt lamps.

The system shown in Figure 7, illustrating the second arrangement of theinvention in its third aspect, is similar to the system shown in Figure3 except that the primary windings W of transformers F are connected inseries with capacitors C and that each capacitor C and its associatedprimary winding W are connected in parallel with the lamp and ballastcombination. The value of the capacitor C is such that when the tap ofthe autotransformer F is set to apply, between the terminals of the lampand ballast combination, a potential difference which is slightly belowthe striking voltage of the lamps, the circuit is resonant at thefrequency of the power source.

When the dimmer control is operated to increase the potential differenceapplied to the lamp and ballast combination, the heating current flowingthrough each electrode is also increased. As the sub-circuit includingthe transformer F approaches the resonant condition, its impedancedecreases until at resonance its impedance reaches a minimum. Thecurrent fiowing through the primary of the transformer F and the heatingcurrent flowing through the electrodes will therefore be of maximumvalue. The potential difference applied across the lamps assists theionisation of the spaces surrounding the electrodes and this ionisationcauses the ends of the lamps to glow. Progressive operation of thedimmer control increases the potential difference applied to the lampand ballast combination until the lamps are at full brightness. When thedimmer control is so operated, the sub-circuit tends to go out ofresonance, its impedance increases and the electrode heating currentstherefore decrease. This decrease is compensated by the heating effectof the discharge which is now established.

This circuit arrangement is particularly suitable for 2-foot -wattlamps. Any number of similar circuits may be connected in parallelprovided that the rating of auto-transformer F is increased to asuitable value and that a separate capacitor is provided for eachprimary winding W of transformer F When used with a 230- volt, SO-cyclepower source, a suitable capacitor had a value of 2 mt. and was ratedfor a maximum working voltage of 440 volts A. C.

The system shown in Figure 8 is similar to the system shown in Figure 7except that each circuit includes a single lamp instead of two lampsconnected in series. This circuit arrangement is particularly suitablefor 80- watt lamps.

The system shown in Figure 9 illustrating the third arrangement of theinvention in its third aspect is similar to the system shown in Figure 3except that the primary winding W of each transformer F is connectedbetween output terminals T and T of a voltage limiting transformer F Theinput terminals T and T of the transformer F are connected to terminalsT and T of the dimmer transformer F Also the second in.- pedance element(i. e. resistor R and capacitor C is connected to the terminal T of thetransformer F instead of to the terminal N of the source, therebyenabling the lamps to be completely extinguished when the dimmer controlis set to zero.

When the dimmer control is operated to increase the potential differenceapplied across the lamp and ballast combination, the potentialdifference applied between the terminals of each electrode is alsoincreased. The circuit is so arranged that, when the value of thepotential difference applied across the lamp and ballast combination isjust less than the striking voltage of the lamps, the electrode heatingcurrents have each reached a value sufficient to enable the lamps tostrike. Under these conditions ionisation occurs around the electrodesand the ends of the lamps glow. Further operation of the dimmer controlincreases the potential difference between the lamp electrodes and thelamps strike, whilst the electrode heating currents are maintainedsubstantially constant by the voltage limiting transformer. The dimmercontrol may then be operated to bring the lamps up to full brightness.Such a voltage limiting transformer is well known to those skilled inthe art.

This circuit arrangement is particularly suitable for Z-foot 40-wattlamps. Any number of similar circuits may be connected in parallelprovided that the ratings of voltage limiting transformer F and dimmertransformer F are increased to suitable values.

The system shown in Figure is similar to that shown in Figure 9 exceptthat each circuit includes a single lamp instead of two lamps connectedin series. This circuit arrangement is particularly suitable for 80-wattlamps.

In the circuit arrangements in which variably-tapped transformers areused these transformers are so connected that the full mains potentialis kept between the lamps and the external electrodes S irrespective ofthe positions of the transformer taps. With such an arrangement smoothvariation in the brightness of the lamps is more readily obtained.

In all the circuit arrangements, when the terminals of the source arerespectively phase and neutral, the ballast impedance is preferablyconnected between the lamps and the phase terminal.

We claim:

1. An electric lamp circuit including two power source terminals and alamp-and-ballast combination having two terminals, the said combinationconsisting of at least one low-pressure electric discharge lamp of thehot-cathode type having two electrodes and connected in parallel with atleast one filament lamp and a ballast impedance con nected in serieswith the said parallel-connected lamps, the circuit also including meansfor supplying heating current from said power source terminalscontinuously to each of said discharge lamp electrodes at least over thelower part of the operating range of brightness of the discharge lampand means for applying a potential difference derived from said powersource terminals between the said two terminals of the combination, thelast-mentioned means including means for varying said potentialdifierence substantially continuously while the discharge lamp isoperating, and the characteristics of the discharge 10 and filamentlamps being so chosen that, in the operating range within which thepotential difierence across the lamp is insufficient to cause thedischarge lamp to strike an appreciable amount of light is emitted bythe filament lamp.

2. An electric lamp system wherein at least two of the lamp circuitsclaimed in claim 1 are connected in parallel and have a common dimmercontrol.

3. An electric lamp circuit as claimed in claim 1, wherein a thirddischarge lamp is connected in series with the said two lamps and asecond impedance element is connected between the junction of the thirdlamp with the other two lamps and a point on the circuit so selectedthat the second impedance element will pass through the said third lampa current sufiicient to maintain the discharge at the lower limit of thecontrollable range of brightness of the third lamp.

4. An electric lamp system wherein at least two of the lamp circuitsclaimed in claim 3 are connected in parallel and have a common dimmercontrol.

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